In a synchronous AC motor, magnetic-pole position needs to be detected in order to determine the phase of the current that flows in each phase coil. For this purpose, magnetic-pole detecting systems in which magnetic-pole position is detected by making each phase current flow in a certain pattern so that the moving element moves to a corresponding stable point are known.
A conventional magnetic-pole detecting system as mentioned above will be described referring to Japanese Laid-Open Patent Publication 1991-89886. According to the patent publication, a magnetic-pole detecting system detects initial magnetic-pole position by changing the phases of an α-phase current Iα and a β-phase current Iβ—which flow in an α-phase coil and a β-phase coil that are formed in the moving element of a 2-phase AC motor—to phases opposite the direction of movement, corresponding to distance traveled by the moving element, so that the moving element moves to a desired stable point.
That is to say, the magnetic-pole position is detected by making the α-phase current Iα and the β-phase current Iβ of equation (4) and (5) described in the foregoing patent publication flow in the α-phase coil and the β-phase coil, so that a predetermined force F (equation (6) described in the above patent publication) is exerted on the moving element so as to move the moving element to a stable point where the force exerted on the moving element is zero.
In such cases, the travel distance ΔX that the moving element moves is given by the following equation (equation (8) in the above patent publication):ΔX=(τ/4−X)/(1+K) wherein
X is the position of the moving element;
τ is the magnetic-pole pitch; and K is a feedback constant when feedback is implemented.
According to this equation, since the travel distance ΔX of the moving element can be reduced approximately in inverse proportion to the feedback constant K, the travel distance ΔX can be shortened by setting the feedback constant K to a larger value.
In this situation, since the moving element is made to move to the stable point by feeding back the position of the moving element, the moving element becomes oscillatory in the vicinity of the stable point, and this oscillation is intensified by setting the feedback constant to a large value. Therefore, by providing, as set forth in the above patent publication, a phase compensator, the oscillation is damped by compensating, in correspondence with the moving speed of the moving element, the current phases, which vary depending on the moving amount that the moving element moves.
However, in the conventional magnet-pole detecting system, in order to inhibit the oscillation of the moving element, a speed detector for detecting the travel speed of the moving element and a phase compensator for compensating the current phases in correspondence with the travel speed of the moving element must be provided, and furthermore the speed feedback value of the moving element has to be adjusted, and thus there has been a problem in that the magnetic-pole detecting system is complex.